1. Field of the Invention
The present invention relates to digital image processing, and more particularly, to a method of removing blocking artifacts from block-coded digital images, and an image reproducing apparatus using the same.
2. Description of the Related Art
Image compression is essential for efficiently processing digital images even when a plurality of memories are necessary. Commonly used standards for image compression are the International Standardization Organization (ISO) standards, such as Joint Photographic Experts Group (JPEG), Moving Picture Experts Group (MPEG)-1, MPEG-2, and MPEG-4, and the International Telecommunication Union (ITU) standards, such as H.261, H.26L, and H.264. Various functions, such as a virtual reality function and an authentication function, are constantly being added to these standards.
According to the image compression standards described above, an encoder for image compression divides a digital image into constant-sized blocks and compresses the divided digital image. That is, the encoder compresses the divided digital image by performing a discrete cosine transform (DCT) and a quantization on a block-by-block basis to obtain a block-coded digital image.
The block-coded digital image is reproduced by an image reproducing apparatus after a decoding process is performed. Since the digital image is processed on a block-by-block basis and information loss frequently occurs in the quantization process, a block boundary is visually distinguished in the reproduced digital image. This is called a blocking artifact.
To remove or reduce blocking artifacts, many methods have been suggested. Generally, the H.264 standard implements a method of removing blocking artifacts using deblocking filters in an encoder and a decoder and removing residual blocking artifacts using a post filter after the decoder.
Hereinafter, for the convenience of description, a block-coded method will be described with reference to the H.264 standard. However, the block-coded method is also commonly used in other image compression standards.
The deblocking filter to remove blocking artifacts in the H.264 standard is defined in a standard specification, according to Equation 1, to control a weight of the filter according to image characteristics.indexA=Clip3(0, 51, qPav+FilterOffsetA)indexB=Clip3(0, 51, qPav+FilterOffsetB)  Equation 1
Here, FilterOffsetA and FilterOffsetB are filtering offsets, and indexa and indexB are determined according to FilterOffsetA and FilterOffsetB. The weight of the filter is obtained by referring to a table defined in the standard specification on the basis of indexa and indexB determined by Equation 1. The weight of the filter may be controlled by adjusting the filtering offsets (FilterOffsetA and FilterOffsetB) to fit the image characteristics. That is, the filtering offsets may be adjusted so that a strong filter is used for a flat image and a weak filter is used to display detailed portions of images beside the flat image.
According to the image compression standards, since the decoder operates in accordance with the encoder, filtering offsets used in the encoder are used as filtering offsets in the deblocking filter. Therefore, the performance of the deblocking filter in the decoder is dependent upon the filtering offsets used in the encoder.
FIG. 1 is a reference diagram illustrating the deblocking filter of the H.264 standard.
Referring to FIG. 1, the filtering offsets (FilterOffsetA and FilterOffsetB) are stored in a slice header 101 and transmitted to a decoder (not shown). That is, the filtering offsets used in an encoder (not shown) are also used in the decoder. Therefore, since the quality of a reproduced digital image may vary according to the filtering offsets used in the encoder, performance of an image reproducing apparatus is dependent on the filtering performance of the encoder.
Also, as shown in FIG. 1, the filtering offsets are defined on a slice-by-slice basis, each slice including a plurality of blocks. Accordingly, in a slice in which a same filtering offset is used, the same filter weight is applied. If image characteristics in the same slice are similar, the blocking artifacts may be efficiently removed. However, if various image characteristics exist in the same slice, since filtering offsets suitable for the various image characteristics cannot be applied, the blocking artifacts cannot be efficiently removed.